Method of oxide-coating aluminum



atented Aug. it, 1936 vii-THO!) F OXIDE-COATING AL No Drawing.Application June 6, i936,

' Serial No. 729,233

This method relates to electrolytic oxide-coating of aluminum andespecially to the method oi making contact in an electrolyte witharticles which are to be given an oxide coating.

In the art of electrolytically treating aluminum, by which term is meantpure aluminum and aluminum base alloys containing '15 per cent or morealuminum, to form on its surface an oxide coating, it is usuallydesirable, in order to conveniently secure a uniform coating over theentire surface, to completely submerge the article in the electrolyteduring treatment, and it therefore becomes necessary to support thearticle beneath the surface of the electrolyte. The electrical circuitis completed through a supporting member which is customarily formed ofaluminum. Some diiiiculty has been encountered heretofore with suchapparatus because the supporting member absorbs an excessive amount ofcurrent from the electrolyte during the treatment, which not onlyshortens the life of the support but also interferes with the propercoating of the article being treated and lowers the electricalefliciency.

It is the object of this invention to overcome these difliculties and toprovide an improved method of supporting articles beneath the surface ofthe electrolyte during treatment in a manner which results in anincrease in operating eiilciency and a decrease in attack upon oroxidation of the work-supporting and contacting means.

This invention is predicated upon the discovery that diflerent aluminumalloys exhibit diiferent capacities for absorbing current fromelectrolytes of the class comprising solutions of sulfuric and oxalicacids. It is believed that this characteristic property cannot beaccurately described in terms generally used in the art because of thefact that the nature of the contact between the surface of the aluminumbeing coated and the electrolyte is substantially different from theordinary electrical contact between the electrode and the electrolyte inan electrolytic plating bath. In the case of aluminum, a coating ofaluminum oxide is formed electrolytically on the aluminum anode surfaceimmediately upon the passage of current. This coating becomes thickerand increasingly firm during the treating operation. The exact manner inwhich the current'penetrates this coating of aluminum oxide, which is anon-conductor of electricity, is not known. However, it has been foundthat aluminum alloys of diiferent compositions and aluminum alloys ofthe same composition but in different conditions, such as heat treated,unheat treated, hard or soft rolled, chill cast or worked, differentcharacteristics in regard to the passage of current. This characteristicproperty is referred to throughout this specification and in theappended claims as the current-absorbing capacity. It has been foundthat if two aluminum alloys possess diflerent current-absorbingcapacities in a given electrolyte at a given temperature, impressedvoltage and other variables of operation, the relation of these twoalloys as to the current-absorbing capacity of each, will remainsubstantially constant for other electrolytes of the class described,and under varying operating conditions. It has been found. further, thatthe attack upon the work-supporting members can be greatly decreased andthe coating process improved by the selection of an aluminum alloy foruse in supporting the workpieces, having a, lower current-absorbingcapacity per unit of area than the article being treated.

In the practical application of this invention, certain compositions ofaluminum alloys, because of their .low current-absorbing capacity, arepreferred for use in forming the supporting contact members. Among thesemay be mentioned aluminum containing silicon as an essential part of thealloy; aluminum containing copper, said alloy being heat treated; andaluminum containing copper and small amounts of manganese and magnesiumor magnesium and silicon, said alloys being in the heat treatedcondition. However, this invention is not confined to any particularaluminumalloys but contemplates broadly the proper combination ofaluminum alloys for supporting or contact-forming materials and for thealuminum or aluminum alloy composition of the article to be supportedfor the electrolytic oxidecoating treatment.

The choice of the aluminum alloy to form a support or contact clamp forany particular aluminum article to be treated may be simply made bypartially immersing a test piece of two materials in a cell containingan electrolyte of the class described and a common cathode or electrode,depending on whether direct or alternating current is used, andmeasuring the amount of current per unit area absorbed from theelectrolyte by each metal. The test may be made by measuring thecurrent-absorbing capacity of each piece individually under identicalconditions of electrolyte, composition, impressed voltage andtemperature. The initial current-absorbing capacity may be somewhathigher or lower than the normal absorbing capacity, thereforemeasurements are made over a short period of time, for example 5 minutesor more, to allow the readings to become constant. The test ispreferably made under conditions identical with conditions contemplatedin operation.

The very substantial difference in the current-absorbing capacity ofdifferent aluminum alloys and the improvement achieved by the usefulemployment of this phenomenon is exemplified by the following tests. Atest piece of hard rolled aluminum sheet composed of aluminum ofcommercial purity containing 5 per cent silicon was placed in a leadcell which also served as the cathode and contained an electrolytecomposed of a 15 per cent solution of sulfuric acid at a temperature of70 F. and under an impressed voltage of 17.5. The current-absorbingcapacity was found by measurement to be 12 amperes per square foot ofarea; while at a voltage of 20.5 the current-absorbing capacity wasincreased to 27 amperes per square foot; and with a voltage of 22.5 thecurrent-absorbing capacity was increased to 45 amperes per square footof area.

Similarly, hard rolled aluminum sheet of commercial purity containing99.2 per cent aluminum was treated under conditions identical with thosein the example given above. At an impressed voltage of 15, thecurrent-absorbing capacity was found to be 12 amperes per square foot;at 17.5 volts, the current-absorbing capacity was 19.8 amperes persquare foot; at 18 volts, 21.9 amperes per square foot; and at 20.5volts, the current-absorbing capacity increased to 63 amperes per squarefoot.

Compared with a current-absorbing capacity of 21.5 amperes per squarefoot for commercial aluminum at an impressed voltage of 18, aluminumcontaining 4.5 per cent copper, 0.8 per cent silicoh and 0.8 per centmanganese having been quenched from a solution heat treatment, treatedunder identical test conditions as the above, absorbed only 9 amperesper square foot at this same potential; at an impressed voltage of 24,the current-absorbing capacity was increased to 12 amperes per squarefoot.

The following examples show the improvement in efficiency made possibleby the application of this invention. Three specimens of aluminum weretested:

(A) One which contained besides aluminum of commercial purity thefollowing elements: copper, 4 per cent; manganese, 0.5 per cent;magnesium, 0.5 per cent.

(B) One which contained besides aluminum, 1 per cent silicon, 0.6 percent magnesium.

(C) Commerically pure aluminum containing less than 1 per cent of theimpurities iron, silicon and copper.

Specimens A and B were in the form of heat treated sheet and C was inthe form of hard rolled sheet. The current-absorbing capacity of thesespecimens was determined as described above, using an electrolytecomposed of a 15 per cent solution of sulfuric acid. The specimensarranged in the order of theircurrent-absorbing capacities were asfollows: A, B and C, A possessing the smallest current-absorbingcapacity and C the largest.

Example 1 A sheet of C aluminum was treated in an electrolyte composedof a 15 per cent solution ofsultrical contact with an aluminum clampformed of B aluminum. After treatment the thickness of the coatingformed on the sheet and on the clamp was measured. The coating on thesheet was found to be 0.00040 inch thick and the coating on the support0.00048 inch. In a similar operation a sheet of A aluminum was heldduring coatingby a support formed of B aluminum. In this case thecurrent-absorbing capacity of the clamp material was greater than theabsorbing capacity of the work held. After treatment the thickness ofthe coating formed on the clamp was found to be 0.00056 inch and thethickness of the coating on the sheet treated was found to be 0.00024inch.

Example 2 A sheet of B aluminum was oxide-coated in an electrolytic bathcomposed of a 7 per cent solution of oxalic acid for a period of 1 hourat a 12 amperes per square foot. The clamp used to support this sheetwas formed of A aluminum having a smaller current-absorbing capacitythan the sheet of B aluminum being treated. The coating formed on thesheet was measured and found to be 0.00083 inch thick while the coatingon the clamp measured 0.00045 inch thick. Similarly treated a sheet of Baluminum was supported by a clamp formed of C aluminum. After treatmentthe coating formed on the sheet in this case measured 0.00080 inch thickand the coating on the clamp measured 0.00083 inch thick.

In Example 1 the use of B aluminum to form a support for'treating Aaluminum resulted in a 16 per cent greater loss of support material thanwhen it was used to support C aluminum or an aluminum having a lowercurrent-absorbing capacity. In Example 2 the efficiency of the coatingprocess was increased by supporting the B aluminum article to be coatedwith A aluminum as compared with'the same process with a support andcontact clamp of C aluminum. In the comparison shown by the firstexample it will also be observed that a much thicker coating is producedon alloy C than on alloy A, and in the comparison shown by the secondexample that there is a much greater attack on alloy C than on alloy Awhen used as clamp materials. This exact relationship may not always befound to obtain, but in every case it will be found that in accordancewith the teaching of the present invention, a given alloy when used as aclamp material will last longer when it has a lower current absorbingcapacity than the articles to be coated. Also, in every case it will befound that a given alloy to be treated can be coated more efliciently byusing a clamp ofan alloy having a lower current absorbing capacity.

It is quite customary in the oxide-coating of aluminum articles to passrelatively large currents through very small electrical contacts betweenthe support and the article being treated.

temperature of 77 F. The current density was 20 ing aluminum surfaces.This method may be 16 applied to the work of a skilled artisan whotreats single, articles to achieve special or unusual results and mustpossess minute control over every detail of his work. It is useful inthe ordinary commercial practice of coating aluminum articles as a meansof increasing the efliciency and decreasing the cost. Possibly thegreatest usefulness of this method lies in its application to theconstruction and design of automatic oxide-coatlng process machineswhich treat large numbers of similar articles with a minimum of laborand expense. Such automatic operations depend primarily for theiradvantages upon uniformity of quality in the product and continuousoperation. By the proper combination, as herein described, of aluminumalloys of difierent current-absorbing capacities in the contacting andsupporting members in the form of supports, clamps, etc., and of thearticle being treated, the periods between stoppage for repairs orreplacements may be greatly lengthened and the quality of the workimproved and/or sustained.

I claim:

1. In a process of electrolytically producing an oxide coating uponaluminum in electrolytes of the class consisting of aqueous solutions ofsulfuric and oxalic acids, the steps comprising determining thecurrent-absorbing capacity of the article to be coated, immersing thearticle to be coated in the electrolyte, and supporting said articletherein by an aluminum support having a predetermined lowercurrent-absorbing capacity than the article supported thereby.

2. In a process of electrolytically producing an oxide coating uponaluminum in aqueous solutions of sulfuric acid, the steps comprisingdetermining the current-absorbing capacity of the article to be coated,immersing the article to be coated in the electrolyte, and supportingsaid article therein by an aluminum support having a predetermined lowercurrent-absorbing capacity than the article supported thereby.

3. In a process of electrolytically producing an oxide coating uponaluminum in aqueous solutions of, oxalic acid, the steps comprisingdetermining the current-absorbing capacity of the article to be coated,immersing the article to be coated in the electrolyte, and supportingsaid article therein by an aluminum support having a predetermined lowercurrent-absorbing capac- 25 ity than the article supported thereby.

HAROLD K. WORK.

